It is well known that in fluid flow control systems volumetric fluid flow rate through a conduit depends on both the flow area of the conduit and the pressure drop thereacross. Accordingly, in establishing a desired volumetric flow rate in such a system, it is necessary to regulate not only the cross-sectional flow area of the conduit, but also the pressure drop therecross. In apparatus such as hydromechanical fuel controls for gas turbine engines, it is desirable to vary engine fuel flow in response to a single input to the fuel control, such input being, for example, movement of a linkage or the like by a corresponding movement of a pilot's power lever. Heretofore, it has been the practice to provide such hydromechanical fuel controls with a throttle (metering) valve and a pressure regulating (bypass) valve which maintains a constant known pressure drop of fuel across the metering valve so that a desired volumetric fuel flow can be established by the pilot's adjustment of the metering valve only. One known type of bypass valve maintains a constant pressure drop across the metering valve by controlling that portion of fuel flow bypassed away from the metering valve from a source of the fuel such as a pump output. One known type of bypass valve includes a spring biased valve element positionable by the pressure drop thereacross of a control fluid (fuel) applied thereto to control the amount of fuel flow bypassed away from the metering valve. This pressure drop across the bypass valve is itself controlled by a differential pressure sensor which detects the pressure drop across the metering valve and a pressure regulator which, in response to an output from the pressure sensor, adjusts the pressure drop across the bypass valve to maintain the desired pressure drop across the metering valve.
Accordingly, it will be appreciated that proper operation of the fuel control depends on the proper operation of the bypass valve, the differential pressure sensor and the regulator. Differential pressure sensors of the type discussed hereinabove frequently employ bellows which respond by linear movement thereof to the sensed pressure while the regulators often employ an orifice accommodating control fluid therethrough and a flapper or similar device to control the effective area of the orifice and thus, the pressure across the bypass valve. It will be readily appreciated that failure of the pressure sensor by a rupture or similar failure of the bellows, or failure of the pressure regulator by fouling of the flapper-orifice assembly by contamination therebetween, will result in a malfunction of the bypass valve and thus, failure of the metering valve to establish the desired flow. In a fuel control for a gas turbine engine powering an aircraft, such a malfunction could result in failure of the fuel control and thus the gas turbine engine itself.
A way of preventing such fuel control and engine failure would be the provision of redundancy in the pressure sensing bellows and pressure regulator flapper-nozzle assemblies. However, such redundancy per se is only achievable at the expense of increased complexity, cost and bulk of the system due to the attendant redundancy in various other collateral parts such as structural and fluid handling components employed with the pressure sensing bellows and pressure regulating orifices and flappers.